The MSG Files, Part II: Natural Sources of Glutamate in Our Bodies and Food

Glutamate plays a role in neurotransmission and cell metabolism, which explains its abundance in our brain and muscles. Source: Journal of Young Investigators

Hi everyone, welcome back to The MSG Files! If you’re just joining us, be sure to read Part I for an introduction to MSG’s definition and history.

Today’s post discusses glutamate in manufactured food additives versus glutamate that’s found naturally in our bodies and food. What foods naturally have the highest amounts of glutamate? What exactly do our bodies do with glutamate? And is there a difference between glutamate that’s found in nature and glutamate in the form of MSG?

As we learned in Part I, there is an entire process behind the commercial production of MSG. However, one does not have to search far to find natural sources of glutamate. Our bodies contain about 10 grams of free glutamate (glutamate that’s not bound to protein) at all times, and produce nearly 50 grams of free glutamate daily. In total, we store around 4 pounds of glutamate (both free and protein-bound) in our muscles and organs. This means that glutamate accounts for more weight in our bodies than our livers or brains do.

Our bodies have the mechanisms in place to detect the presence of glutamate in our food. In the early 2000s, nearly a century after Kikunae Ikeda first hypothesized umami to be the fifth basic taste, a series of studies confirmed that we do indeed have umami-specific taste receptors.

The vagus nerve responds to the presence of glutamate in our stomachs, and prepares our body for protein digestion. Image from McGraw-Hill

In 2000, a study published in Nature identified the first of these umami taste detectors – a receptor called “taste-mGluR4″. Two years later, scientists showed that a coupling of two receptors, T1R1 and T1R3, creates a composite amino-acid receptor that also contributes to umami recognition; another study published that same year strongly supported this claim. Then, in 2005, scientists found one more receptor, mGluR1, that plays a role in the umami response mechanism. Together, these three receptors — taste-mGluR4, T1R1+3, and mGluR1 — facilitate the gustatory sensations of umami.

Glutamate receptors are present, not just on our tongues, but also in our stomachs. The vagus nerve, our line of communication between the stomach and brain, specifically recognizes glutamate. When glutamate hits receptors in the tongue and stomach, the vagus nerve alerts the brain, prompting our gastrointestinal tract to begin protein digestion.

Glutamic acid enantiomers, L-glutamic acid and D-glutamic acid, are mirror images of one another. L-glutamic is found in animal and plant protein, and is responsible for the umami taste.

Glutamic acid occurs in two forms, or enantiomers, that are mirror images of one another: L-glutamic acid and D-glutamic acid. L-glutamic acid is the amino acid typically found in animal and plant protein. It breaks down to form L-glutamate, which produces the umami taste. D-glutamic acid, on the other hand, is found primarily in the cell walls of certain bacteria.

Fermented foods tend to have high levels of D-glutamate relative to foods with added MSG or nonfermented foods with naturally-occuring glutamate. In our food, just as in our bodies, glutamate is either bound to, or free of, proteins. Our taste receptors detect free L-glutamate, which is the dominant form in MSG.

We start ingesting glutamate at an early age – it is present in both amniotic fluid and human breast milk, which contains glutamate at concentrations similar to that of broth, and around ten times that of cow’s milk – but our glutamate consumption doesn’t end there.

Aside from the kombu that Kikunae Ikeda enjoyed, foods with notable amounts of free glutamate include cheeses (particularly roquefort and parmesan), cured meats, tomatoes, scallops, peas, onions, asparagus, spinach, mushrooms (especially if they’re dried), and green tea. Free glutamate also forms in the production of foods like soy sauce, worcester sauce, and the oft-loved/oft-hated British spread, Marmite. Bound glutamate, which is abundant in high-protein foods like meat, fish, and milk, gets broken down into free glutamate during digestion.

Certain processes enhance the amount of free glutamate in food. One such process is ripening. In fruits and vegetables, glutamate reaches its peak concentration when the umami taste is optimal. For instance, as tomatoes transition from crisp green to bold red, their glutamate concentration increases ten-fold. When cheeses age, free glutamate increases as proteins break down. This explains why mature cheeses have particularly potent flavors.

Natural processes aside, we can also increase glutamate levels when we handle food. When we cure meat or fish, we cause proteins to break down and give off free glutamate. Lastly, cooking releases free glutamate in savory foods.

All told, the average person consumes a quotidian 10-20 grams of glutamate, most of which is bound to the proteins in our food. Free glutamate from seasoning or condiments represents a relatively small fraction of our glutamate consumption — some estimates put the value at 10%. Since our bodies already produce it, most of the glutamate from our diet just gets metabolized as a source of energy in the gut. Furthermore, there is no difference in the way our bodies process glutamate from MSG and glutamate that is found naturally in our food. Regardless of source, L-glutamate undergoes the same digestion and absorption processes.

At this point, the obvious question to ask is: if our body doesn’t differentiate between the glutamate in MSG and the glutamate that’s found naturally our bodies and foods, what is the big deal with MSG? Stay tuned to find out! Later this week, we’ll be discussing Chinese restaurant syndrome — where and when it first cropped up, what symptoms it entails, and how it brought notoriety to everything MSG touched. But first, we’ll be taking a whirlwind tour through the elusive fifth taste — umami.